Hi Pegabo,
If the gel is 4 lb density then it’s a good quality material and wouldn’t be a weak link in the mattress.
While they didn’t provide the density of the bottom layers … assuming that they are at least 1.8 lb density then it would certainly be reasonable to expect a useful life in the range of 10 years (regardless of the length of the warranty) and it’s possible that you would have some “bonus time” after that as well.
I would keep in mind that a warranty only covers manufacturing defects and doesn’t cover the loss of comfort and support that can come from foam softening and isn’t an indication of how long it may be before you may need to replace it (see post #174 here).
It sounds like you made a good quality choice … and congratulations on your new mattress 
Phoenix